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The present invention relates to foamed gypsum products and more particularly to a mixer for preparing a foamed gypsum slurry for use in making foamed gypsum products.
Gypsum wallboard is a common building material. It generally is made of a hardened gypsum plaster, i.e. a mixture of calcium sulfate and water, sandwiched between two facing sheets, typically paper. It is common to mix foam with the gypsum plaster to produce a product having less weight. Various systems have been developed for making foamed gypsum wallboard with a hard edge, see for example U.S. Pat. No. 4,279,673 issued to White et al. on Jul. 21, 1981. In the White patent a first rotary mixer is used to mix calcined gypsum, water, and foam and other rotary mixers are used to defoam portions of the output of the first mixer to provide a more dense plaster for making the hard edges.
Efficient use of foam reduces costs and can improve finished product quality. As a result, efforts have been made to more efficiently mix foam with a gypsum plaster to reduce the amount of foam generating chemicals used, see for example U.S. Pat. No. 5,484,200 issued to Bradshaw on Jan. 16, 1996. In the Bradshaw patent, one rotary mixer is used to mix calcined gypsum and water, and a second mixer is used to mix foam with the slurry produced in the first mixer.
It would be desirable to provide a system for efficiently mixing foam into a gypsum slurry without requiring the use of additional energy required by secondary rotary mixers.
The present invention provides an apparatus for mixing an aqueous foam with a calcined gypsum and water slurry. The apparatus includes a cylindrical mixing chamber having a slurry diverter at a first end and a circular constriction at a second end. The chamber has an inlet near its first end for receiving a flow of calcined gypsum and water slurry between its central axis and outer wall. The diverter directs the flow in a spiral or helical path. Aqueous foam is injected into the inlet, preferably through the diverter, and mixes with the slurry by turbulence as the slurry spins in the chamber. The circular constriction causes further turbulence to further mix the foam with the gypsum slurry.